Supported reagents have gained increased use during recent years and have found wide application in the fields of organic and inorganic chemistry, biochemistry, and biology. In particular, supported alkali metal borohydrides, especially sodium borohydride, have been shown to possess great value as reducing agents. The use of such reagents leads to many advantages over the use of unsupported reagents. For example, the effective reaction area and rate is increased and the entropy of activation lowered. Other advantages of supported reagents include: simplified reaction work-up because the spent reagent can usually be removed by simple filtration, greater selectivity than the corresponding homogeneous reaction, milder reaction conditions, and the ability to recover and recycle reagents. In the case of supported sodium borohydride reagents, it has been observed that the by-product of its use in chemical reductions, sodium borate, is substantially retained upon the support, thereby minimizing contamination of the reaction solvent with boron-containing residues.
E. Santaniello, F. Ponti, and A. Manzocchi, Synthesis, 891, Dec. 1978 reported a technique for adsorbing an aqueous solution of sodium borohydride onto alumina followed by drying to yield a powdered reagent containing about 10% sodium borohydride. Such reagent is useful to reduce aldehydes and ketones in non-aqueous solvents such as ether, benzene, or ethylacetate, in yields over 80%. Normally, sodium borohydride reduction in these solvents is impractically slow. Other examples of supported borohydride reagents are given by F. Hodosan and N. Suban, Rev. Roumaine Chim., 14, 121 (1969); E. Santaniello, C. Farachi, and A. Manzocchi, Synthesis, 912 (1979); and V. Ciurdaru and F. Hodosan, Rev. Roumaine Chim., 22, 1027 (1977).
The prior method of preparing supported alkali metal borohydride reagents has been very cumbersome, dangerous and, extremely wasteful of the expensive borohydride reagent. This method involves preparation of an aqueous solution of the borohydride reagent which is then added to a dry support such as alumina or silica with good mixing, followed by vacuum evaporation of the majority of the water, followed by more complete water removal by storing the reagent in a P.sub.2 O.sub.5 dessicator for several days. Only very small quantities of the reagents can be prepared with this procedure. Thus such procedure is unsatisfactory when attempting to prepare supported reagents on a larger scale. When aqueous solutions of sodium borohydride are added to an anhydrous alumina support the reaction becomes extremely hot, hydrolysis of the sodium borohydride takes place causing decomposition of a substantial part of the expensive reagent and also causing rapid evolution of copious and hazardous amounts of hydrogen. The hydrogen then must be safely removed from the reaction zone. The hydrolysis reaction is: EQU NaBH.sub.4 +2H.sub.2 O.fwdarw.NaBO.sub.2 +4H.sub.2
Also, the resulting product is contaminated with large amounts of the hydrolytic decomposition product, sodium borate. In attempts to prepare sodium borohydride on silica on a kilogram scale, the hydrolysis reaction has become so violent that explosions have occurred, and it consequently, became impossible to use the aqueous process. Also, all of the NaBH.sub.4 was lost due to hydrolysis.